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Cold seeps and cold-seep carbonates

Rights: University of Waikato. All Rights Reserved. Published 5 October 2012Download

Dr Steve Hood explains that a cold seep is the expulsion of hydrocarbon-rich fluids at the seafloor. Over time. a specialist type of limestone can form. These are quite unlike shelf limestones found on land. Ancient cold-seep limestone deposits have been found on land in the East Coast region of the North Island.

Points of interest

Look for the following micrographs:

Micrograph a: Thin section image as seen under cathodoluminescent light of cold-seep limestone. The worm tube cross-section is infilled with radiating needles of aragonite crystals that have a blue colour.

Micrograph b: Thin section image as seen under cathodoluminescent light of cold-seep limestone. Needles of aragonite crystals growing off a bivalve shell fragment.

Transcript

DR STEVE HOOD A cold seep is the expulsion of hydrocarbon-rich fluids at the seafloor, and the temperature of these seeps is relatively consistent with the ambient or surrounding water, so they’re not like hot vents that you get in relation to subduction volcanics further to the north around Kermadecs.

The limestones that form around cold seeps are a specialist type of limestones, quite unlike your shelf limestone that you get around Waitomo/Te Kuiti area. These are much smaller in their extent, and their origin is very much related to the seepage of hydrocarbons that have migrated from deep down below the seafloor. Hydrocarbons, being light, move up, migrate through fractures, faults, and end up percolating through the sediment on the seafloor.

Modern cold-seep carbonates are forming on the seafloor today, off the east coast of eastern North Island, and much of the work that we’ve done here at Waikato and Auckland Universities is studying some of the ancient cold-seep deposits or limestones that are uplifted now from the seafloor on land in east coast North Island New Zealand, and they date back from, you know, about 20 million years and significantly younger.

The interesting thing with the minerals that are precipitated in a cold-seep environment is they’re quite different or unique from your typical shelf limestone sequence that you see in Te Kuiti and around New Zealand. These minerals that form in these seep limestone tend to be aragonitic. We get tiny little fine crystals of aragonite cement growing that act to bind and glue all the dead fossil material together, thereby creating a limestone.

So we know where we’ve got cold-seep carbonates, limestones that are formed in the past that clearly there was an association with hydrocarbons. Now some of these hydrocarbons could be being sourced from gas hydrates, which are frozen methane deposits below the seafloor, and these are an invaluable or potentially mineable future resource.

Acknowledgements: Dr Jens Greinert, Royal Netherlands Institute for Sea Research National Institute of Water and Atmospheric Research (NIWA) Jens Greinert et al., 2010; Marine Geology; doi:10.1016/j.margeo.2010.01.017 Associate Professor Kathleen Campbell, University of Auckland